This invention relates to lead frames for rectangular flat packages having lead terminals which extend outwardly through the four peripheral sides of a resin body.
In a conventional lead frame 1 for a rectangular flat package shown in FIG. 3, the lead frame includes a central portion 3 on which a semiconductor element 2 is to be mounted and a plurality of lead terminals 4 distributed around the central portion 3 and extending toward the four peripheral sides of the frame. The inner ends of the lead terminals 4 terminate adjacent to the central portion 3 which is supported by two central portion leads 5. The lead terminals 4 and the central portion leads 5 are coupled together partway along their length by dam bars 6 and they are divided into four groups corresponding to the four peripheral sides of the frame 1. The lead terminals 4 and the central portion leads 5 are also joined at their outer ends to peripheral buffer portions 7 separated from the rest of the frame by slits 8. Two side rails 10 extend along the edges of the frame 1 and have guide holes 9 at preselected intervals so that the guide holes can be used, for example, to position the frame. In the arrangement shown in FIG. 3, the lead frame 1 is coupled to adjacent lead frames in a strip by section bars 11.
A rectangular flat package is made from a lead frame 1 as described hereinafter. The semiconductor element 2 is first bonded onto the central portion 3 of the frame 1 and the terminals of the element 2 are then connected by wires to the lead terminals 4 of the frame. An epoxy resin or similar material is molded, for example, by transfer molding, around the lead frame 1 so as to make a resin body 12 in an oblong area inside the dam bars 6 as shown by a dot-dash outline in FIG. 3. The semiconductor element 2 is thus encapsulated by the resin body 12. Thereafter, the dam bars 6 are punched out and the lead terminals 4 are cut off and bent.
When the epoxy resin or similar material thermally hardens during molding to make the resin body 12, the molded material contracts. Since the lead frame 1 has a lower coefficient of thermal expansion than the molded material and is very thin, the frame is likely to undergo warp or deformation as the resin body contracts. However, because of the presence of the slits 8, the buffer portions 7 are deformed to allow the resin body 12 to contract in the longitudinal and transverse directions without causing the lead frame 1 to undergo much warp or deformation as a whole.
Nevertheless, the conventional lead frame 1 has a problem as described hereinafter. The resin body 12 of the rectangular flat package contracts toward its center in all directions, but the deformation of the buffer portions 7 of the lead frame does not prevent the contraction of the resin body in the diagonal directions from causing the lead frame to undergo warp or deformation. For that reason, the frame 1 cannot smoothly be conveyed during the punching and bending operation after the manufacturing of the resin body 12.